Trapped and interfacial charges have significant impact on the performance
of organic light-emitting devices (OLEDs). We have studied devices consisti
ng of 20 nm copper phthalocyanine (CuPc) as the buffer and hole-injection l
ayer, 50 nm N,N'-di(naphthalene-l -yl)-N,N'-diphenyl-benzidine (NPB) as the
hole transport layer, and 65 nm tris(8-hydroxyquinolinato)aluminum (Alq(3)
,) as the electron transport and emitting layer sandwiched between a high-w
ork-function metal and a semitransparent Ca electrode. Current-voltage meas
urements show that the device characteristics in the negative bias directio
n and at low positive bias below the built-in voltage are influenced by tra
pped charges within the organic layers. This is manifested by a strong depe
ndence of the current in this range on the direction and speed of the volta
ge sweep. Low-frequency capacitance-voltage and static charge measurements
reveal a voltage-independent capacitance in the negative bias direction and
a significant increase between 0 and 2 V in the given device configuration
, indicating the presence of negative interfacial charges at the NPB/Alq(3)
interface. Transient experiments show that the delay time of electrolumine
scence at low voltages in these multilayer devices is controlled by the bui
ldup of internal space charges, which facilitates electron injection, rathe
r than by charge-carrier transport through the organic layers. To summarize
, our results clearly demonstrate that the tailoring of internal barriers i
n multilayer devices leads to a significant improvement in device performan
ce.